All-season pneumatic tires for passenger vehicles are required to excellent performance on snow when running on a snow-covered road surface as well as wet performance or wear resistance when running on a road surface not covered with snow (wet road surface or dry road surface).
In order to enhance wet performance, silica is typically blended into a rubber composition for a tire. However, in comparison to carbon black, the reinforcement performance when blended into a diene rubber is small, which leads to the problem of reduced wear resistance. In addition, when the compounded amount of silica is increased or the particle size thereof is made fine in order to enhance wet performance, there are problems in that the dispersibility of the silica decreases and the wear resistance is further diminished, or the flexibility of the rubber composition is lost and the performance on snow is diminished.
To resolve this problem, Japanese Unexamined Patent Application Publication No. 2009-91498A and International Patent Application Publication No. WO/2011/105362 propose enhancing the dispersibility of silica with a rubber composition in which silica is compounded in a terminal-modified styrene-butadiene rubber having terminals modified with a polyorganosiloxane or the like. However, the required level anticipated to enhance wet performance and performance on snow is even higher, and there is a demand to further enhance these characteristics.